1. Field of the Invention
This invention relates to a dual fuel combustion low NOx combustor of a gas turbine.
2. Description of the Related Art
In recent years, various improvements have been made on a combustor, etc. in a gas turbine to decrease NOx and raise the temperature of the gas turbine (raise the inlet temperature of the turbine), thereby achieving a high efficiency.
As shown in FIG. 4, for example, a fuel F, which has been injected through a pilot nozzle 102 provided at the center of a combustor inner tube 101, and a plurality of main nozzles 103 provided around the pilot nozzle 102, and compressed air PA, which has been discharged from a compressor 104 and introduced to an upstream side of the combustor inner tube 101, are mixed in a combustor 100 of a gas turbine. Then, the mixture is combusted in a combustion zone on a downstream side of the combustor inner tube 101 or an upstream side of a combustor transition pipe 105, and is introduced as a high temperature, high pressure combustion gas CG into the turbine equipped with stationary blades 106 and moving blades 107. In the turbine, the combustion gas CG is expanded to serve as a driving force, which drives the compressor 104 and outputs a surplus driving force to the outside.
The ratio between the compressed air PA and the fuel F introduced into the combustor inner tube 101 (i.e., fuel-air ratio) needs to be controlled to take an optimal value according to the operating state of the gas turbine (namely, the amount of fuel charged). For this purpose, not all of the compressed air PA is introduced into a combustion section of the combustor 100, but part of the compressed air PA is bypassed and flowed from a turbine casing 108 into the combustor transition pipe 105. A bypass valve 109 is provided for this purpose, and allows part of the compressed air PA to be flowed and supplied into the combustor transition pipe 105 from an opening portion of a bypass pipe 110 provided in the turbine casing 108.
In such a combustor 100, the upstream side of the combustor inner tube 101 is allocated as a first stage combustion zone, and the downstream side of the combustor inner tube 101 is allocated as a second stage combustion zone. A relatively small amount of fuel is injected through the pilot nozzle 102 into the first stage combustion zone to generate a high temperature combustion gas. With this combustion gas as a flame (trigger), a large amount of a lean premixed fuel mixture is injected through the main nozzles 103 into the second stage combustion zone, whereby the generation of a locally high temperature combustion gas is prevented, and NOx is kept to a minimum (see, for example, Japanese Patent Application Laid-Open No. 2000-130756).
As the combustor 100 mentioned above, a so-called dual fuel combustion low NOx combustor is known which has the pilot nozzle 102 capable of injecting a gaseous fuel and a liquid fuel simultaneously or selectively, and the plurality of main nozzles 103 disposed around the pilot nozzle 102 and being capable of injecting a gaseous fuel and a liquid fuel simultaneously or selectively.
The pilot nozzle 102 is taken as an example for illustration, as shown, for example, in FIG. 5. The pilot nozzle 102 has a liquid nozzle portion 112 of a pressure spraying type, provided at the center of a nozzle body 111, for spraying a liquid fuel, and a plurality of gas nozzle portions 113, concentrically surrounding the liquid nozzle portion 112, for injecting a gaseous fuel obliquely outwardly.
In such a dual fuel combustion low NOx combustor, the pilot nozzle 102 renders varieties of fuels available, and can use different fuels in combination, thereby actualizing diffusive combustion with excellent stability of combustion, while the main nozzles 103 can use many fuels, thereby making it possible to decrease the amount of a pilot fuel used in diffusive combustion, and achieve pre-mixed combustion involving a minimal NOx concentration (see, for example, Japanese Patent Application Laid-Open No. 1997-264536).
In the above-described dual fuel combustion low NOx combustor, however, a pressure spraying type nozzle is used as the liquid nozzle portion 112 for injecting a liquid fuel in the pilot nozzle 102. This has posed the problem that if an operation with a high pilot ratio (a high ratio of the amount of the liquid fuel injected from the pilot nozzle 102 to the amount of the liquid fuel injected from the main nozzles 103) is performed to ensure combustion stability, for example, during a light load operation of the gas turbine, smoke (black smoke) occurs, causing pollution.